Power-control system for stirling engines

ABSTRACT

A power-control system for a Stirling engine which includes a supply line by which working gas is added to the engine&#39;&#39;s compression space to increase pressure therein and power, and a dump line for discharging gas and reducing power, and a storage container having two variable-volume chambers divided by a bellows; the first chamber contains a quantity of said working gas maintained at a substantially constant temperature which is in communication with said supply and dump lines via appropriate valves, and the second chamber is sealed and contains an azeotropic mixture such as methane and butane. When the gas is added to the first chamber, the bellows moves to reduce the volume of the second chamber, with a conversion of some gas to liquid in the second chamber, and maintenance of the pressure in both chambers.

United States Patent 11 1 .laspers 1 1 June 3, 1975 1 POWER-CONTROLSYSTEM FOR [731 Assignee: North American Philips Corporation, New York.NY.

22 Filed: July 22.1974

21 App1.No.:490,728

Primur Evwniner-Martin P. Schwadron Assistant Evaminer-H. Burks. Sr.Attorney. Agent. or F irmFrank R. Trifari [57] ABSTRACT A power-controlsystem for a Stirling engine which includes a supply line by whichworking gas is added to the engines compression space to increasepressure therein and power. and a dump line for discharging gas andreducing power. and a storage container having two variable-volumechambers divided by a bellows; the first chamber contains a quantity ofsaid working gas maintained at a substantially constant temperaturewhich is in communication with said supply and dump lines viaappropriate valves. and the second chamber is sealed and contains anazeotropic mixture such as methane and butane. When the gas is added tothe first chamber, the bellows moves to reduce the volume of the secondchamber, with a conversion of some gas to liquid in the second chamber,and maintenance of the pressure in both chambers.

8 Claims, 6 Drawing Figures Patented y} June 3, 1975 4 Sheets-Sheet 1Fig.

l mDOmOP SPEED Patented June 3, 1975 3,886,744

4 Sheets-Sheet 2 Fig. 3

Patented June 3, 1975 4 Sheets-Sheet Fig.5

Patented June 3, 1975 [ATM] Pressure 4 Sheets-Sheet 4 VALVE 39 MOVES EASTROKE,BOTTLE 4O STROKE,BOTTLE 4| BELLOWS DISPLACEMENT Fig. 6

POWER-CONTROL SYSTEM FOR STIRLING ENGINES BACKGROUND OF THE INVENTIONThis invention concerns a control system for regulating the power outputof a Stirling engine by varying the pressure of the working gas in theengines working spaces. Typically this is accomplished by connectingsupply and dump lines or gas conduits to the engines compression space,with valves arranged to permit gas flow into the compression space forincreasing pressure therein which results in higher output torque and topermit gas flow out of the compression space for reducing pressure andtorque. The gas will flow merely by having the supply line at higherpressure than the compression space which cycles between high and lowcycle pressure. and having the dump line at lower pressure. Apparatusfor the above system typically includes a storage container holding aquantity of working gas at high pressure, a compressor having its intakecommunicating with the engines dump line and its discharge communicatingwith the storage container, and a supply line from the container to thecompression space.

While Stirling engines may be used to drive various devices such as boatpropellers, automobiles, and electric power generators, the operationcharacteristics of the driven devices are not always readily compatiblewith the Stirling engine, such that certain combinations have beenconsidered to be non-feasible, which is the subject of the presentinvention.

FIG. 2 of the drawings illustrates certain relevant characteristics ofthe subject devices, relative to a torque vs. speed diagram. Curve a-bis a typical torque curve for a driven device such as a boat propelloror an automobile. For higher speeds or power, greater torque isrequired, with the power demanded, P Torque x Speed X Constant. Curvec-b is the torque curve for another driven device, an electric powergenerator wherein speed is shown to be constant at level c;substantially constant speed is a requirement in order to provide stablefrequency and voltage, regardless of the load on the system and thecorresponding power demanded by the generator. In this constant speedapplication power will vary only with torque, since Power Torque xConstant speed x Constant. Furthermore, torque x is directlyproportional to pressure in the working space; high pressure curve P3corresponds to high torque e, as compared to low pressure curve P1relating to lower torque f. Accordingly for a given increase in power,torque (as affected by pressure) must increase more greatly with fixedspeed than where speed also increases. Thus, for electric powergeneration with a Stirling engine, operation over a substantial pressurerange will be required.

The use of a compressor-storage tank control system for regulation overa wide pressure range has various undesirable features. A hermetic sealmust be maintained throughout the working gas system, including thecompressor, the storage tank, the supply and dump lines. and the workingspace. In order to accommodate the high pressure range, the storage tankmust be very high pressure, i.e. 250 at; and the compressor intake mustrelate to the low pressure. i.e. 50 at. while its discharge must relateto the high pressure. 250 at. Thus a complex and expensive compressor isrequired. that must operate over a large pressure range: the hermeticseal problem is heightened due to high temperature in the compressorfrom the extreme pressure range, and also there is noise, vibration, andpower drain caused by the compressor. The present invention provides acontrol apparatus which substantially overcomes all of' theabove-described undesirable features.

SUMMARY OF THE INVENTION The invention is a new power regulating systemin combination with a Stirling engine. The working gas space of theengine is in communication via a dump line and a supply line andappropriate valves with a storage container of working gas. Thiscontainer includes a bellows separating a closed space containing abinary liquid-vapor system, and a second space containing working gaswhich can be conducted to and from said working space via said supplyand dump conduits respectively. The liquid-vapor system is preferably anazeotropi'c mixture, such as methane and butane which will develop apressure greater than 100 atmospheres at room temperature ofapproximately F. With this container structure the working gas supply ismaintained at a substantially constant pressure regardless of whethergas is added to or removed from the gas space within the container. Byusing two such containers at different high and low pressures, it ispossible to control the power of a Stirling engine over a large pressurerange, such as 3:1, even while maintaining constant engine speed, as isrequired in electric power generation systems. With this inventionnumerous advantages result as compared to prior art control systemswhich utilize a compressor and a plain storage container having only acavity for working gas, and optional spring means tending to maintainthe gas pressure even when same has been discharged. More specificfeatures and advantages of the invention are developed in thedescription of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of aconventional powercontrol system with a Stirling engine.

FIG. 2 is a diagram plotting the relationship of speed vs torquepressure and power for a typical Stirling engine.

FIG. 3 is a schematic view of a power control system of the inventionwith a Stirling engine.

FIG. 4 is a schematic view of a storage container of the invention.

FIG. 5 is a schematic view similar to FIG. 3, with two storagecontainers of gas at high and low pressures respectively.

FIG. 6 is a schematic view showing bellows displacement relative topressure, for a power control system using two storage containers athigh and low pressures respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT A conventional power controlsystem shown in FIG. I shows the working space 10 of a Stirling enginecommunicating with a supply line 11 and a dump line 12. Check valve 13permits gas flow into space 10 when the gas pressure therein drops belowthe pressure of gas in the supply line; check valve 14 permits gas flowout of space 10 when the pressure therein exceeds the dump linepressure. The gas flow in dump line 12 can be directed via valve back tothe supply line 11, or via valve 16 to the compressor 17 whose intake 18has the lowest pressure of the system. By operation of the compressorthe gas pressure is raised sufficiently high to recharge the storagecontainer 19 from the compressor discharge 20. Valve 17 allows highpressure gas from the container to flow via supply line 1 1 back toworking space 10. The variation of mean pressure experienced by theworking space is 50 to 150 atmospheres. while the pressure rangeexperienced by the compressor between intake and discharge would be 65to 250 atmospheres, the latter pressure being the maximum storagecontainer pressure. A hermetic seal 21 is maintained about the workinggas system, including the compressor, the working space, and the storagecontainer. Automatic control means 22 opens and closes valves 15, 16,and 17 as required.

FIG. 2 provides various torque curves showing the relationships ofspeed, torque, pressure, and power for a Stirling engine and for certaintypes of driven devices, this Figure having been discussed above in theBackground of the Invention.

A basic embodiment of the invention is shown in FIG. 3 where a Stirlingengine working space 23 has its pressure varied via supply and dumplines 24 and 25 respectively, using valves 26, 27, 28, 27a and 28a, asin FIG. 1. However, container 29 is totally different from the simplepressurized tank 19 of FIG. 1; this new container 29 is shown in detailin FIG. 4, where within heat-insulated housing 30 is a bellows 31separating gas space 32 from liquid-vapor space 33. A binary liquidvaporsystem is established by using mixture of methane and butane, such as 54percent liquid methane 46 percent liquid butane, which will provide atroom temperature of about 70F (21C) a pressure of 108 at. This is aconvenient and useful temperature and pressure range, as opposed tousing water-water vapor mixture, which would require a temperature ofover 300C to develop the desired pressure over 100 at.

In operation chamber 32 can supply gas at a substantially constantpressure despite variations in the quantity of gas present, because asgas is discharged via port 34 to the supply line 24, and when thepressure in space 32 tends to drop, the vapor pressure in space 33 willurge the bellows to move and reduce the volume of space 32 which willbring the pressure there back up, while in space 33 additional liquidwill vaporize to bring the pressure therein back to nominal. A layer ofgauze 35 or equivalent on the inside wall of chamber 33 hastensvaporization of the mixture when there is a rapid increase in volume ofspace 33. To protect the bellows from overtravel should the container bedischarged excessively, a support 30a is provided to arrest movement ofthe bellows beyond the supports end surface; on contact of the bellowswith this support the container opening becomes sealed to preventfurther gas discharge. To further hasten vaporization an electric heater30b may be incorporated into the containers wells 30, but such is notnecessary for the basic operation.

In operation of the engine, when supply line valve 28 is opened, gas athigher pressure from chamber 32 will flow to engine compression space23; and when dump line valve 27 is opened gas can flow from the space 23into chamber 32, with corresponding movement of the bellows into space33, and condensation of vapor therein. A single container 29 as shown inFIG. 3 is adequate for a Stirling engine to operate with a 2:1 pressurevariation in the working space.

Where a higher pressure change, such as 3:1 is required. the embodimentof the invention shown in FIG. 5 may be used. Here a Stirling engineworkspace 36 communicates via supply and dump lines 37, 38 through valve39 with two storage containers 40, 41 similar to that of FIG. 4, but attwo different high and low pressures respectively, such as andatmospheres. The high pressure container will be operable with the dumpline, and the low pressure container with the supply line, with valve 39operable automatically to select the appropriate container. The workingspace 36 has a variation of mean pressure between 50 atm and I50 atm,and FIG. 6 shows schematically the pressure cycle by which the valve 39switches the connection between the two bottles.

Upon command from the electronic control 42 to decrease power, valve 43is opened. At this instant, the pressure in the dump line 38 is themaximum cycle pressure, 207 atm, since the engine is at full meanpressure. Simultaneously, valve 44 is opened, changing the pressurephase, while gas flow through valve 43 lowers the mean pressure in theengine by gas transfer to the storage system, until the desired lowpower output level is reached. With the pressure level in the dump lineat 207 atm and the pressure level in container 40 at 110 atm, helium istransferred through the dump valve into container 40 as indicated inFIG. 6. Moving from left to right in FIG. 6, the maximum cycle pressureor dump line pressure decreases until it almost equals the 1 10 at.pressure in container 40. To descend to still lower pressures valve 39switches the dump line to container 41 while sealing off container 40.Now dump line at a pressure slightly above H0 at. communicates withcontainer 41 whose pressure is 65. Gas transfer continues into container41 until the maximum cycle pressure has decreased to about 65 atm. Thenthe bypass 44 and dump 43 valves are closed, and the mean cyclepressure, as indicated in FIG. 6, has decreased from atm to 50 atm, apressure change of 3:1.

To raise the pressure, and through it the power output, the process issimilar except that the operation is performed on the supply line 37through valve 45. Starting at the lowest mean pressure, valve 45 isopened connecting container 41 at 65 atm to the supply line 37 which isat the minimum cycle pressure of 35 atm. Gas is thus transferred fromcontainer 41 into the supply line and into the engine. This continuesuntil the minimum cycle pressure approaches 65, at which time valve 39switches the supply line to bottle 40 at 110 at; then pressure risesuntil the minimum cycle pressure is about 110, with the maximum cyclepressure at 207.

The response time in going from full power to idle will be about 0.1 see(with the aid of the bypass valve 44). After about 5.0 sec, gas transferto the storage system will be complete and the bypass valve closed. Theresponse time in going from idle to full power is estimated to be 0.5sec.

The invention as described above has numerous im portant advantages overthe prior art control systems using a compressor and a plain storagetank. With the elimination of the compressor. we greatly reduce cost.noise, maintenance and seal problems of this relatively complexmechanism; the result is a cheaper more reliable control-system, whichoperates with lower gas storage pressures.

A further advantage of this invention concerns sudden stoppage of theengine while operating with high pressure in the working space. in priorart control systems, with a compressor hermetically sealed into theworking gas system, a stopped engine at high pressure remained in thatcondition, and subsequently it was extremely difficult to overcome thispressure upon the compression piston when trying to re-start the engine.With the present invention the above-described problem will not occur.At any time the dump line valve can be opened to one of the storagecontainers; regardless of whether the engine is running. The quantity ofmixture in the storage container space 33 is selected such that uponfull extension of the bellows into space 33, substantially all of themixture will be condensed, and the bellows will then be supported byliquid and protected from damage due to overextension.

I claim:

1. In a Stirling engine which includes a working space where a workinggas is cycled between high and low pressures, the improvement incombination therewith of a power regulating system comprising a storagecon- 7 tainer with a bellows therein separating first and secondchambers, the first chamber containing working gas, the second chambercontaining a binary liquid-vapor mixture, the power-regulating systemfurther comprising a supply conduit for conducting gas from said firstchamber to said working space, and a dump conduit for discharging gasfrom said working space to said first chamber, and valve means forselectively regulating said gas flows in said supply and dump conduits,said gas in the first chamber being maintained at a substantiallyconstant pressure since said mixture in the second chamber will vaporizeand condense respectively according to the decrease or increase of gasin the first chamber.

2. Apparatus according to claim 1 wherein said liquid-vapor systemcomprises an azeotropic mixture of methane and butane.

3. Apparatus according to claim 2 wherein said mixture comprises 54percent liquid methane and 46 percent liquid butane by molecular weight.

4. Apparatus according to claim 2 wherein said mixture will have apressure greater than 100 atmospheres at room temperature ofapproximately F.

5. Apparatus according to claim 1 wherein said container is a generallycylindrical housing with a closed end and an open end defining a gasport therein, said first chamber communicates with said port, and saidsecond chamber is bounded by said closed end and said bellows.

6. Apparatus according to claim 5 wherein said bellows is movablebetween an extended position into said second chamber when working gasis added to the first chamber and a compressed position into said firstchamber when working gas is discharged from said first chamber, andwherein said container further comprises support means in said firstchamber and adjacent said bellows for restraining movement of thebellows beyond said compressed position, during excessive discharge ofworking gas.

7. Apparatus according to claim 6 further comprising means for closingsaid port when said bellows moves to its compressed position.

8. Apparatus according to claim 1 further comprising a second workinggas storage container similar to said first container, the liquid-vaporsystem of one container providing a working gas pressure at roomtemperature substantially greater than the other system, and controlvalve means for selectively connecting said dump conduit and supplyconduit to the higher or lower pressure container.

=l l l

1. In a Stirling engine which includes a working space where a workinggas is cycled between high and low pressures, the improvement incombination therewith of a power regulating system comprising a storagecontainer with a bellows therein separating first and second chambers,the first chamber containing working gas, the second chamber containinga binary liquid-vapor mixture, the power-regulating system furthercomprising a supply conduit for conducting gas from said first chamberto said working space, and a dump conduit for discharging gas from saidworking space to said first chamber, and valve means for selectivelyregulating said gas flows in said supply and dump conduits, said gas inthe first chamber being maintained at a substantially constant pressuresince said mixture in the second chamber will vaporize and condenserespectively according to the decrease or increase of gas in the firstchamber.
 1. In a Stirling engine which includes a working space where aworking gas is cycled between high and low pressures, the improvement incombination therewith of a power regulating system comprising a storagecontainer with a bellows therein separating first and second chambers,the first chamber containing working gas, the second chamber containinga binary liquid-vapor mixture, the power-regulating system furthercomprising a supply conduit for conducting gas from said first chamberto said working space, and a dump conduit for discharging gas from saidworking space to said first chamber, and valve means for selectivelyregulating said gas flows in said supply and dump conduits, said gas inthe first chamber being maintained at a substantially constant pressuresince said mixture in the second chamber will vaporize and condenserespectively according to the decrease or increase of gas in the firstchamber.
 2. Apparatus according to claim 1 wherein said liquid-vaporsystem comprises an azeotropic mixture of methane and butane. 3.Apparatus according to claim 2 wherein said mixture comprises 54 percentliquid methane and 46 percent liquid butane by molecular weight. 4.Apparatus according to claim 2 wherein said mixture will have a pressuregreater than 100 atmospheres at room temperature of approximately 70*F.5. Apparatus according to claim 1 wherein said container is a generallycylindrical housing with a closed end and an open end defining a gasport therein, said first chamber communicates with said port, and saidsecond chamber is bounded by said closed end and said bellows. 6.Apparatus according to claim 5 wherein said bellows is movable betweenan extended position into said second chamber when working gas is addedto the first chamber and a compressed position into said first chamberwhen working gas is discharged from said first chamber, and wherein saidcontainer further comprises support means in said first chamber andadjacent said bellows for restraining movement of the bellows beyondsaid compressed position, during excessive discharge of working gas. 7.Apparatus according to claim 6 further comprising means for closing saidport when said bellows moves to its compressed position.